I’ve used Dagger and Koin while working at Yelp and Hootsuite in enterprise-scale Android projects, and played around with Kodein in personal projects, and wanted to compare and contrast using the three different libraries. This cheat sheet allows you to quickly see what the different Hilt and Dagger annotations do and how to use them. We’ll define a Provides function in a module, allowing Dagger to know this is the instance we want injected. This isn’t meant to be a complete setup tutorial, since lots of those exist for all three libraries already, but I’ll provide some links to good existing ones. Our ComponentHelper looks like this. Go check its usage in the Google I/O 2020 app and in the dev-hilt branch of the Android architecture-samples Github repository. I’ve had some luck asking in the #kodein channel in the Kotlin Slack; some of the most active Kodein contributors are in the channel and give detailed responses to people’s questions there. If you have thoughts on the Android View Model or want to share your approaches to manual DI, don’t hesitate to reach out @ajkueterman on Twitter, or follow me on DEV.to and leave a comment on this story. Dagger has been the reigning dependency injection library in Android for a long time, but a few alternatives have appeared recently. Here’s the official Android quickstart documentation and a helpful tutorial. The good news is above website gives the step to step guides, check out the Setup section. Note: If you want more information about dagger I can highly recommend Hari Vignesh Jayapalan introduction to dagger2 posts where he goes from the basics to more advanced dagger topics in just a few posts. A factory can take up to five arguments. We used Constructor Injection in our AppModule when providing our repository, as seen on line 10 of the first code sample. Similarly, If you wanna write agile and maintainable code in a large and long-term run Android project, Dagger should be an not so bad option. Yeah, Dagger is not just for Android, but for any Java Project. We use the provides annotation if there is no constructor we can inject from and when we can’t instantiate the dependency. If you’re just curious about the new annotations and what you can do with Hilt, check out this cheat sheet in the section below. By following DI principles, you lay the groundwork for good app architecture, greater code reusability, and ease of testing. We would like to inject that repository into our View Model when we construct it. Hilt is currently in alpha, try it in your app and give us feedback using this link. One way to enable this behavior is to use ViewModelProvider.Factory, with which you can instantiate a View Model with it’s needed dependencies. It’s a promising future, but only available from API 28. * Get a [ViewModel] in a [Fragment]. Out of the three, Kodein has the slowest runtime performance. Testing on Kodein seems relatively straightforward but unfortunately, there’s not much official documentation about it and I can’t tell how difficult it would be to use in larger, more complex projects. Run the code again, and it all works! Hope you found this exploration useful! Finally, I should point out that all three can peacefully coexist in the same project. They’re less widely used than the three I covered, but are actively maintained and would be fun to check out and experiment with. We can use this to easily inject our framework classes in a scalable way. Nothing much has changed. The Winner of the Vice Presidential Debate? The test dependencies are set up in a structure similar to the Fibonacci sequence, to simulate multiple levels of transitive dependencies. But once you run into the exception, you’ll be able to see a stacktrace and know exactly which line caused it. However, an interesting inclusion in Android 9 Pie may have just changed this entirely. We can’t use Context instances here, as our instantiate functions are being called before context exists — confusing! The House class, which can access Koin’s inject() method once it implements the KoinComponent interface: We have to declare a Koin module for providing the dependencies: MainApplication has to implement the KoinComponent interface as well, and call startKoin() in its onCreate(): The House class needs to implement the KodeinAware interface, and also declare a value for kodein: Again, we’ll need to declare a module for providing the dependencies: MainApplication has to implement the KodeinAware interface and declare a value for kodein: It feels boilerplate-y to have to override kodein in every KodeinAware class, although you can use the closestKodein() function inside an Android class or closestKodein(context) function in a non-Android one to retrieve your Kodein object.

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